Page 11 - Vol_1_Chapter_VI

10
Fig. 6. 6 Nitriding subtypes

6.3.2 Salt Bath Nitriding

Process description
Salt bath nitriding is a thermochemical process in which nitrogen and carbon are diffused
simultaneously into the surface of the material.
Before nitriding, the components have to be thoroughly cleaned and degreased. Any surface
contamination from grinding particles, oil or metal chips will result in an uneven formation of the
nitrided layer. This can cause cracks in the coating which leads to flaking and corrosion. After cleaning,
the parts are dried and preheated and then transferred to the actual nitriding environment. The
various nitriding processes can be differentiated mainly by their nitrogen source and the energy supply.
A typical commercial bath is composed of a mixture of 60-70% sodium salts {96.5% NaCN, 2.5%
Na2CO3, 0.5% NaCNO} and 30-40% potassium salts {96%KCN, 0.6%K2CO3, 0.75% KCNO, 0.5% KCl} (Fig.
6. 7). 11

Benefits
The high concentration of nitrogen chemically combines with iron and other nitride forming elements
to produce an outer layer of epsilon iron nitride (Fe3N) which is thin, hard and ductile. An advantage
of the salt bath nitriding process is that it’s carried out at lower temperatures that keep parts in the
ferritic phase, leading to reduced risk of distortion during treatment.

Application
This process is a surface treatment for most ferrous metal components and is designed to improve
certain engineering properties to enhance wear resistance, lubricity, fatigue strength and corrosion
resistance (except for stainless steel). This nitriding process will also lower the coefficient of friction
and possibly reduce the material cost of components i.e., low alloys versus high alloys.

Copyright © by DIGIGREEN All Rights Reserved Volume I - Chapter VI: Digital and Green Surface Conditioning 11